1. Field of the Invention
The present invention relates to a data storage method, and more particularly relates to a magnetic disk control device provided with cache memory in which data is read/written according to instructions from a host computer in association with a magnetic disk system using a disk array.
2. Description of the Related Art
Lately disk devices, such as magnetic disk devices, optical disk devices and the like, that are characterized by the non-volatility and large capacity of stored data have been widely used as the external storage devices of a computer. As a disk device which duplicates the contents of data and stores data adding redundant information, such as parity data and the like, a redundant array of inexpensive disk (RAID) device is used.
In both such a RAID device and a RAID control device controlling the transfer of data, for example, between the RAID device and a host computer, data transfer performance is improved by mounting a large capacity of cache memory and temporarily storing data in the cache memory. As the amount of data to be handled increases, a huge number of various RAID devices, specifically, in the range of level from RAID 0 through RAID 5 are connected and used under the control of such a control device.
FIG. 1 explains a conventional example of a cache data management method in such a RAID control device. In FIG. 1, data located in an MRU position that is most frequently used by a host computer up to data a LRU position that is least frequently used are arrayed in the cache memory of the RAID control device, using a link or the like. If there is an access from the host computer, it is regarded that there is a cache hit, and the computer is connected to the MRU position as the most frequently used data.
In FIG. 1, write-back, that is, asynchronous writing into a disk device is applied to data written from the host computer into the cache memory. This write-back into the RAID device (disk) is applied from the least frequently accessed data, that is, data located in the LRU position by an LRU method, taking into consideration holistic performance as a data storage system.
However, write performance from the cache memory into the RAID device (disk) varies depending on a RAID type (level). Specifically, at RAID 0+1 and RAID 5 at which data is distributed to and written into a plurality of disk devices, since the plurality of disks operates in parallel, throughput performance is high. Conversely, at RAID 1 in which data with the same contents is written in two disk devices, since its performance cannot exceed the performance of a disk, its throughput performance is low.
If data to be written in a plurality of RAID devices each with different write-back performance is managed by data transfer through one segment of cache memory, write data into a device with a low write-back performance of RAID 1 is easily left in the cache memory. After the cache memory is filled with such left data, access performance to RAID 0+1 and RAID 5 also is affected by write-back performance into the disk device with RAID 1 and degrades, which is a problem.
As the prior art of such a RAID data storage system, there are the following references.    <Japanese Patent Laid-open Application No. 11-288387 “Disk Cache Device”>    <<Japanese Patent Laid-open Application No. 2003-196032 “Write Cache Control Method of Storage Device and Storage Device”>>
<Japanese Patent Laid-open Application No. 11-288387 “Disk Cache Device”> discloses a disk cache device in which valid data is prepared on a cache block so that RAID 3 access in which parity can be calculated, for example, in RAID 5 operation, without a disk lead, is possible in order to optimize a disk cache function.
<<Japanese Patent Laid-open Application No. 2003-196032 “Write Cache Control Method of Storage Device and Storage Device”>> discloses a technology for improving throughput using a write cache controlling method capable of executing sequential data write without fail even when the addresses to be written designated by a host device are not consecutive in a write-back cache type storage device.
However, in such prior arts, if devices with a variety of levels are mixed as RAID devices, holistic performance as a data storage system is affected by a RAID device with low write-back performance and degrades.